1. Field of the Invention
The present invention relates to a display device, a liquid crystal display device, an electronic apparatus, and a display device manufacturing method. More specifically, the present invention relates to a display device and the like provided with a touch sensor that is capable of detecting positions touched by a finger or a pen.
2. Description of the Related Art
A touch sensor is a device for detecting positions touched by a finger or a pen, and it is normally used in combination with a display device such as a liquid crystal display device (LCD), a plasma display device (PDP), or the like.
The touch sensor is utilized to one of man-to-machine interfaces, such as a keyboard, a mouse, and the like of a computer. The output signals of the touch sensor are generated by touching directly prescribed positions such as button images on a screen of a display device with a contact body such as a human finger or a pen to the computer and the output signals control an apparatus or to control display contents of the display device. The display with the touch sensor (hereinafter referred to as a touch panel) are currently put into practical use in portable information terminals, ticket-vending machines, automated teller machines (ATM), car navigation systems, copying machines, and the like.
An analog capacitance coupling type, a resistance film type, an infrared type, an ultrasonic type, and an electromagnetic inducing type are known as the types of the touch sensor. Among those, the analog capacitance coupling type is further classified into a projected capacitive type and a surface capacitive type.
The surface capacitive type touch sensor is configured with a transparent substrate, a uniform transparent conductive film formed thereon, and a thin insulating film formed thereon.
When driving the touch sensor, an AC voltage is applied from the four corners of the transparent conductive film. When the touch sensor is touched by a finger, a small electric current is flown in the finger because of a capacitance formed between the touched surface and the finger. The electric currents flow from each corner of the film to the touched point.
Then, a controller obtains the ratio of the electric currents from each corner and calculates the coordinates of the touched position. Regarding the technique of the surface capacitive type touch sensor, Japanese Examined Patent Publication 56-500230 (Patent Document 1) discloses a basic concept device.
However, when the AC voltages are simply applied to the transparent conductive film from the four corners, an electric field lines distributed on the transparent conductive film becomes curved and non-linear. Therefore, the touched positions by a finger or a pen cannot be detected accurately. Techniques for overcoming such issue are disclosed in Patent Document 1 and Japanese Patent No. 3121592 (Patent Document 2).
Those Documents mention about linearization patterns provided on the outer periphery of a transparent conductive film. An electric field is extended from the linearization pattern provided in each side of the outer periphery towards the counter side, and the electric field intensity becomes constant in a direction perpendicular to the respective sides of the outer periphery. That is, regarding the potential distribution on the transparent conductive film, equipotential lines in parallel to each side of the outer periphery are formed, and the pitches of the equipotential lines become uniform and linear. Therefore, the relation between the potential distribution on the transparent conductive film and the corresponding finger touched positions can be simplified.
Patent Document 1 discloses a structure which superimposes conductive segments on a resistive surface (referred to a conductive impedance surface hereinafter) by silk screening printing technique (from the seventh line to the twelfth line of lower left section of p. 5 of Patent Document 1, and FIG. 5 of Patent Document 1).
Further, also disclosed is an example which arranges conductive segments in a geometric pattern, and repeatedly corrects the system of an isochrone equation showing the resistance net between the segments to obtain the optimum geometric shape (from the thirteenth line to the eighteenth line of lower left section of p. 5 of Patent Document 1, and FIG. 6 of Patent Document 1).
With the above, the degrees of the current density and the directions thereof at any points on a plane become uniform on the conductive impedance surface, thereby providing a resistive surface which generates linear electric fields.
Further, Patent Document 2 discloses an example which provides linearization patterns along the edges of a position detecting conductive film (hereinafter referred to as a position detecting film) of a counter substrate side for making electric field lines uniform (paragraph 0017 of Patent Document 2, FIG. 5 of Patent Document 2). Further, in the example of Patent Document 2, a layer of a position detecting film is formed on the counter substrate, and a layer of linearization patterns is further formed on the layer of the position detecting film (FIG. 7 of Patent Document 2).
In the meantime, the latest technology trends regarding the analog capacitance coupling type are disclosed in “Technologies and Developments of Touch Panels” supervised by Yuji MITANI, CMC publishing CO., LTD. Dec. 1, 2004, pages. 54-64 (Non-Patent Document 1). For the touch sensor of the analog capacitance coupling type of a related technique, a touch sensor of a surface capacitive type formed on a transparent substrate is attached on a display device to be used.
However, such structure has the touch sensor provided further on the display screen, so that there are some issues to be overcome, e.g., an increase in the thickness of the device itself, an increase in the cost, and deterioration of the display quality by putting the touch sensor on the display. Techniques for overcoming such issues are disclosed in Japanese Unexamined Patent Publication 2003-99192 (Patent Document 3) and Japanese Unexamined Patent Publication 2003-66417 (Patent Document 4).
Patent Document 3 discloses a device which calculates position coordinates of a touched position based on electric currents at the four corners by having current detectors provided the four corners on a counter electrode surface which applies voltages to a liquid crystal.
Patent Document 4 discloses a device which includes: a liquid crystal display (LCD) circuit for supplying display voltages or currents to a transparent conductive film; a position detecting circuit for detecting electric currents flown from a plurality of points of the transparent conductive film; and a switching circuit for making a counter electrode and one of those circuits electrically conductive.
Further, Patent Document 4 discloses an example which uses the counter electrode surface to function as a position detecting conductive film (transparent conductive film) (0030 of Patent Document 4), and position detecting electrodes for detecting positions of applied voltages are formed at the four corners of the position detecting conductive film. Furthermore, Patent Document 4 discloses an example which forms a plurality of position detecting electrodes along the periphery of the counter electrode surface.
With the techniques of Patent Documents 3 and 4, the common (COM) electrode or the transparent conductive film for LCD drive circuit functions as the transparent conductive film of the surface capacitive type touch sensor, so that it is unnecessary to additionally provide a surface capacitive type touch sensor to the display device. As a result, it is possible to overcome the issues such as an increase in the thickness of the device itself, an increase in the cost, and deterioration of the display quality.
However, there are still remained issues with the display devices disclosed in Patent Documents 3 and 4 as follows.
That is, while Patent Documents 3 and 4 are described to utilize the counter electrode surface as the transparent conductive film for detecting positions in order to overcome the issues such as the increases in the thickness of the device itself and the cost, there is no depiction about a specified structure and method for forming the linearization patterns.
Therefore, the display devices disclosed in Patent Documents 3 and 4 are suited for reducing the weight, the size, and the thickness, but not capable of correctly detecting the positions touched by a finger or a pen.
In the meantime, the method for forming the linearization patterns according to the related technique is a screen printing of a conductive paste, etc., as depicted in Patent Document 2. For such method, it is necessary to add a step for forming the linearization patterns, and to provide a special manufacturing device such as a screen printing device.
Further, the conductive paste for the linearization patterns is normally made with fine powders of noble metals such as silver, so that such expensive material is required.
As a result, the cost for manufacturing the display devices is increased if the techniques of Patent Documents 3 and 4 and the technique of Patent Document 2 are simply combined in order to form the linearization patterns on the counter electrode.